Composite containers for packaging products under pressure, particularly, refrigerated dough products, constitute a significant commercial consumer product. Typically, the containers are formed of a spirally wound paperboard or boardstock layer; an interior liner which is a laminate including kraft paper, foil and/or polymer layers; and an exterior label. The spirally wound boardstock layer includes an unglued butt joint extending helically from end to end of the can. The exterior label surrounds the boardstock and covers the butt joint thereby preventing the premature opening of the butt joint.
Commercially significant containers of this type are disclosed in U.S. Pat. No. 3,981,433 to Thornhill et al. which is directed to one step easy-open containers including an inner liner having a helical fold positioned opposite the unbonded helical butt joint. When the outer wrapper is removed, the dough and the liner expand outwardly together as the liner fold or pleat begins to unfold. The resultant pressure on the can body causes the butt joint to open. This in turn automatically allows the inner liner to expand further. The liner is automatically opened by the rapidly expanding dough to thereby allow access to the dough in the interior of the container.
The outer label surrounding the butt joint in containers of this type is an important structural component of the container because the outer label bridges the butt joint and maintains it in the closed position. Accordingly, the label must be strong in order to prevent premature opening of the container. Such structural requirements for the label prevent the use of more economical low strength sheet materials for construction of the label.
It is generally recognized that vacuum packaging with easy-open containers of the butt joint type can only be accomplished with difficulty, if at all. Because of the structural design of the composite container, the application of vacuum to the interior of the container often results in partial or complete inward collapsing of the container walls along the butt joint seam. This can result in an unacceptable appearance for the composite container or in unacceptable sealing of the product within the container.
The manufacture of butt jointed easy-open containers is accomplished by a process including the step of spirally winding boardstock about a mandrel. The winding operation must be carried out in a highly precise manner so that edges of the boardstock are in continuous contact, but not overlapped, along the entire length of the mandrel. Boardstock width must also be uniform or there will be gaps between adjacent edges of the boardstock wrapped around the mandrel.
Numerous modifications have been proposed for easy-open spirally wound composite containers. For example, an easy-open container having a spirally wound lap joint is proposed in U.S. Pat. No. 3,221,975 to Zoeller et al. The overlapping edge of the joint is permanently glued to the underlying ply. A plurality of discontinuous perforations are provided in the overlapping seam so that the overlapping seam, itself, forms a tear strip for opening of the container.
U.S. Pat. No. 4,235,341 to Martin et al. proposed a spirally wound container having a thin paper base stock wrapped with an inner ply of film adhering to the surface of the base stock to add strength to the composite container. The container was said to include a spiral seam which could be perforated if desired but which was said to be in most cases an overlapped glued seam which would burst when the outer ply was removed and the container was twisted open or banged against a hard edge. The inner ply surrounding the base stock was said to tear with the body ply along the spiral seam when the container was twisted or banged against a hard surface.
Numerous other proposals have been made for modifying and improving easy-open spirally wound containers particularly in view of the consumer preference for these containers and the convenience associated with them. Nevertheless, no substitute easy-open container for dough and similar products has been commercialized having the benefits and capabilities associated with spirally wound butt jointed easy-open containers while overcoming deficiencies associated therewith.